1. Field of the Invention
The invention relates to a high density read-only memory (ROM) cell, and more particularly, to a high density ROM cell having a structure similar to a metal oxide semiconductor (MOS) transistor.
2. Description of the Prior Art
As semiconductor technology progresses, circuits manufactured by semiconductor technology are widely used in electronic devices and computer systems. One example of these circuits is a ROM. The prior art combines MOS transistors with peripheral circuits to form ROMs. However, each of theses ROMs can only store one bit data. Since circuit area corresponds to the number of MOS transistors and peripheral circuits, a high capacity memory circuit will occupy a large area on a semiconductor chip. As a result, high capacity memory circuits cannot be satisfactorily integrated on a single wafer.
Please refer to FIG. 1 showing a circuit diagram of a conventional ROM cell, wherein each MOS transistor is a ROM. As described above, each ROM cell can store only one bit of data; the data can be read under the control of word lines WL0, WL1, WL2, WL3 . . . (four word lines are shown in FIG. 1) and bit lines BL0, BL1 . . . (two bit lines are shown in FIG. 1).
As described above, in the conventional memory circuit, each ROM cell can store only one bit of data, and the circuit area corresponds to the number of these transistors and peripheral circuits. Thus, a high capacity memory circuit occupies a large area on a semiconductor chip, which means that high capacity memory circuits cannot be satisfactorily integrated on a single wafer. Accordingly, the cost of a high capacity memory circuit cannot be reduced, and the capacity of ROM is hardly improved. Thus, the prior art retains disadvantages which can be improved upon.